Electroluminescent device



ELECTRODE STRIPS Z4 PHOTOC'ONDUC'T/VE [AYER M. WASSERMAN ELECTROLUMINESCENT DEVICE Filed July 14, 1959 Z2 l6 7 4 l4 INVENTOR g MOE WASSERMAN BY w Dec. 25, 1962 ELECT/30' ATTORNEY ite Efilfififil Patented Dec. 25, 1962 3,976,791 ELEiITROLUll/HNESCENT DEVICE Moe Wasserman, Massapeqna Park, N.Y., assignor to Sylvania Electric Products Ellth, a corporation of Dela- Ware Filed duly T14, 1959, Ser. No. 827,111 11 Claims. Cl. 250-209) My invention relates to information storage and display devices employing a plurality of bistable electroluminescent cells.

A bistable electroluminescent cell comprises separate photoconductive and electroluminescent layers electrically connected in series. The electrical characteristics of the two layers are chosen such that the photoconductive impedance in the dark is high relative to the electroluminescent impedance. Further, when the photoconductive layer is illuminated, the photoconductive impedance is significantly reduced. The photoconductive and electroluminescent layers are so arranged that some of the light emitted from the electroluminescent layer, upon excitation of the latter (by application of a voltage across the series connected layers), impinges on the photoconductive layer.

As a consequence, when a voltage is applied between the two layers and the photoconductive layer is in the dark, the electroluminescent layer remains quiescent and emits substantially no light. The cell is then in its first, or unexcited, electric state.

However, when a voltage is applied between the two layers and the photoconductive layer is stimulated by a light signal, the impedance of the photoconductive layer decreases sharply; consequently, a larger portion of the applied voltage appears across the electroluminescent layer and light is emitted. The cell is then in its second, or energized, state. As long as the photoconductive layer remains illuminated by light from the electroluminescent layer, the cell will remain energized in the presence or absence of incident light. conventionally, the cell is eenergized by removal of the applied voltage.

As is known to the art, bistable electroluminescent cells can be interconnected to form a matrix wherein each cell can be separately placed in either one of its two states, thus permitting information to be simultaneously stored and displayed.

1 have invented a new type of information storage and display device wherein the electroluminescent and photoconductive components are not separate adjacent cells as heretofore required, but in contradistinction, are in the form of continuous layers. Further, in my device, the bistable cells are formed by varying the geometry of electrical conductors and-opaque coatings associated with the continuous layers, thus permitting a significantly simplified construction of devices having large numbers of small bistable elements of controllable geometry.

In accordance with the principles of my invention, my device includes first and second vertically separated electrode sets. A photoconductive layer is placed between the electrode sets in such manner that one surface of the photoconductive layer is in contact with one electrode set. A network or" separated transparent conductors is applied over the other surface of the photoconductive layer. An electroluminescent layer is applied over this network. An opaque film having a plurality of separated transparent areas, each of which is in registration with a corresponding conductor, is interposed between the electroluminescent layer and the other electrode set. (Alternatively, the opaque film can be interposed between the network of conductors and the photoconductive layer instead of the position indicated above.)

The opaque film and the network of conductors thus divide the continuous electroluminescent and photoconductive layers into a plurality of bistable electroluminescent cells, each of which is normally in the dark or unexcited state. Any cell can be energized by applying an appropriate voltage between selected electrodes in the first and second sets and, at the same time, irradiating the cell with light. Any cell, when energized, will remain illuminated upon removal of the incident light, and can only be extinguished by removing (or sharply reducing) the applied voltage.

Illustrative embodiments of my invention will now be described with reference to the accompanying drawings wherein:

FIG. 1 is a cut away isometric view of one embodiment of my invention;

FIG. 2 is a cross sectional view of another embodiment of my invention; and

FIG. 3 is a cross-sectional view of still another embodiment of my invention.

Referring now to FIG. 1, there is shown an image storage and display device having a glass substrate 10. A first electrode set, which in this example consists of horizontal parallel separate electrode strips 12, is applied over one surface of the glass substrate it An electroluminescent layer 14 is applied over electrode strips 12. A plurality of transparent conductor or electrode elements, which can have any convenient shape, as for example squares 1-6, is applied over the electroluminescent layer 14. The electrode squares 16 are arranged in rows and columns, the rows being in registration with corresponding electrode strips 12. An opaque film 18 is applied over the electrode squares. The opaque film has a plurality of light transparent areas 29, each of which is in registration with the corresponding electrode square 16. Each transparent area is substantially smaller than that of the corresponding electrode square. A photoconductive layer 22 is applied over the opaque layer 18. Finally, a second electrode set, which in this example consists of vertical separate electrode strips 24, is applied over the photoconductive layer 22. The columns of electrode squares are in registration with corresponding electrode strips 24.

Each of the electrode squares 16 in combination with the portion of the structure above and below it constitutes a bistable electroluminescent cell which is energized and deenergized in the manner previously described.

Alternatively, the opaque film lfi can be removed from the position shown in FIG. 1 and, instead, can be interposed between the electroluminescent layer 14 and the electrode strips 12, as shown in FIG. 3. Further, if desired, two such films can be used together, one film being in the position shown in FIG. 1, the other film being interposed between the electroluminescent layer 14 and the electrode strips 12.

The opaque filrn of films serve to eliminate crosstalk, i.e., to avoid accidental light triggering of one bistable cell by an adjacent bistable cell. It should be noted that, by virtue of the light transparent area 20 of the opaque fiilm 18, the central portion of each electrode square is left uncovered to permit optical feedback between the electroluminescent and photoconductive layers associated with the appropriate cell.

When the bistable elements, as determined by the electrode geometry, are closely spaced, the single opaque film shown in PEG. 1, is sometimes not adequate to prevent cross-talk. Under these circumstances, additional optical isolation between adjacent elements can be obtained by applying squares 26 of a conductive opaque material between the electroluminescent layer and electrodes 12, as shown in FIG. 2, the squares being smaller in area than the electrode squares 16 with which squares 26 are in registration. Alternatively, the conductive squares 26 can be interposed between the electrodes 12. and the glass substrate or can be embedded within the electroluminescent layer 14.

In the arrangement thus far described, electrical energy is supplied to any cell by applying a voltage between the horizontal and vertical electrodes secured to the cell. Alternatively, the second electrode set can take the form of a common or continuous electrode which contacts one surface of all cells. Further, if desired. the first electrode set can take the form of separate electrodes as, for example, electrode squares which each contact only one cell.

The ratio of the dark cell impedance to the lighted cell impedance can be adjusted by varying the electrode width ratio of the vertical electrodes 24 to the width of the electrode squares 16. Typical ratios fall within the range 0.25-0.50. Further control of this ratio can be obtained by varying the thickness of the photoconductive layer to that of the electroluminescent layer.

An illustrative process for preparing a device as shown in FIG. 2 is as follows:

A fiat piece of window glass is covered with a transparent tin oxide coating, which is subdivided by appropriate graphic arts techniques into the first set of parallel strips, 0.042 inch wide, spaced 0.205 inch apart (l6 strips per inch). A square array of opaque gold squares, 0.026 inch on a side, spaced 0.0365 inch apart, is superimposed. An electroluminescent layer composed of a dispersion of electroluminescent (copper-activated zinc sulfide) particles in a glass frit is then applied by spraying or similar technique; the la er is dried at 110 C. to remove water and volatile solvents and fired using a time-temperature cycle which fuses the frit and secures a non-porous glass enamel lacquer coating the phosphor. (A typical thickness for the electroluminescent layer is about 1.52.5 mils.) A transparent tin oxide electrode is then applied over the electroluminescent layer. This is subdivided by appropriate graphic arts techniques into a square array of squares, 0.042 inch on a side, with 0.0205 inch separation, in register with the array of gold squares. A thin black glass enamel film is then applied, containing square holes, 0.026 inch on a side, in register with the gold squares.

The photoconductive layer comprising cadmium sulfide particles activated with copper and coactivated with a chloride and dispersed in a glass frit is then applied in the manner set forth in my copending patent application Serial No. 796,155, filed February 27, 1959 (Docket #8910). (Typically, the photoconductor layer is thicker than the electroluminescent layer, the ratio of thicknesses normally falling with in range 3-5.) A transparent tin oxide coating is then applied over the photoconductive layer. This oxide coating is subdivided into the second set of parallel strips, 0.011 inch wide, with 0.515 inch spacing, extending in the direction normal to the set of strips on the glass base.

The device so produced has the following characteristics. When excited by 400 volts R.M.S. at 3000 cycles per second, the brightness in the lighted state of the visible areas is approximately 10 foot-lamberts. Using as triggering source an incandescent lamp of color temperature 2800 K, an exposure of approximately 0.3 foot candle seconds is sufficient to place an element into the lighted state. This is equivalent, for example, to a 3 millisecond exposure at 100 foot candle illumination. The voltage range between the minimum for maintaining the lighted state and the point at which self-triggering takes place is approximately 100 volts. This range is great enough so that an operating voltage can be chosen at which selective erasure of lighted elements may be accomplished without critical control of the voltage.

Thus, for example, the device may be operated at a point 60 volts above the minimum threshold value without self-triggering occurring. By reducing the voltage on a particular row by 40 volts and on a particular column by 40 volts, the element at the intersection will be at 20 volts below its threshold, and become extinguished while all other elements will be at 20 volts above their threshold, and any which are lighted will remain in that state.

If desired, the electroluminescent layer can be formed by two successively applied electroluminescent films. Under these circumstances, the gold squares can be applied over the first electroluminescent film, and the second electroluminescent film can be applied thereover, thus embedding the squares in the composite electroluminescent layer. Alternatively, the first electroluminescent film can be completely coated with an opaque non-conducting film, thus providing two electroluminescent films which are optically independent; the first film providing visual display, the second film forming an element of the bistable cells.

What is claimed is:

1. An electroluminescent device comprising first and second spaced apart electrode sets; an electroluminescent layer interposed between said sets; a photoconductive layer interposed between said electroluminescent layer and said second set; a network of separated transparent conductors interposed between said photoconductive and electroluminescent layers; and an opaque film having separated transparent areas, each area being in registration with a corresponding transparent conductor, said opaque film being interposed between said network and said photoconductive layer, said layers, said network and said film being electrically connected in series between said sets.

2. A device as set forth in claim 1 wherein each of said electrode sets consists of separated conductors, each electrode set conductor being in registration with a corresponding conductor in said network.

3. A device as set forth in claim 1 wherein said second electrode is a single common electrode.

4. An electroluminescent device comprising first and second spaced apart sets of parallel separated electrodes, the first and second set electrodes extending in nonparallel directions; an electroluminescent layer interposed between said sets; a photoconductive layer interposed between said electroluminescent layer and said second set; a network of separated transparent conductors interposed etween said photoconductive and electroluminescent layers, a separate transparent conductor being positioned at each point whereas an electrode in one set crosses over an electrode in the other set; and an opaque film having separated transparent areas, each area being in registration with a corresponding transparent conductor, said opaque film being interposed between said network and said photoconductive layer, said layers, said network and said film being electrically connected in series between said sets.

5. An electroluminescent device comprising first and second spaced apart sets of parallel separated electrodes, the first and second set electrodes extending in non-parallel directions; an electroluminescent layer interposed between said sets; a photoconductive layer interposed between said electroluminescent layer and said second set; a network of separated transparent conductors interposed between said photoconductive and electroluminescent layers, a separate transparent conductor being positioned at each point Whereat an electrode in one set crosses over an electrode in the other set; and an opaque film having separated transparent areas, each area being in registration with a corresponding transparent conductor, said film being interposed between said electroluminescent layer and said first set, said layers, said network and said film being electrically connected in series between said sets.

6. An electroluminescent device comprising first and second spaced apart sets of parallel separated electrodes,

the first and second set electrodes extending in non-parallel directions; an electroluminescent layer interposed between said sets; a photoconductive layer interposed between said electroluminescent layer and said second Set; a network of separated transparent conductors interposed between said photoconductive and electroluminescent layers, a separate transparent conductor being positioned at each point whereat an electrode in one set crosses over an electrode in the other set; and an opaque film having separated transparent areas, each area being in registra' tion with a corresponding transparent conductor, said film being interposed between said network and said photoconductive layer, said layers, said network and said film being electrically connected in series between said sets.

7. An electroluminescent device comprising first and second spaced apart electrode sets; an electroluminescent layer interposed between said sets; a photoconductive layer interposed between said electroluminescent layer and said second set; a first network of separated transparent conductors interposed between said photoconductive and electroluminescent layers; and an opaque film having separated transparent areas, each area being in registration with a corresponding transparent conductor, said opaque film being interposed between said first network and said photoconductive layer; a second network of separated opaque conductors, each opaque conductor being in registration with a corresponding transparent conductor, said second network being interposed between said electroluminescent layer and said first set, said layers, said networks and said film being electrically connected in series between said sets.

8. An electroluminescent device comprising an electrically insulating transparent substrate; first and second spaced apart sets of parallel separated electrodes arranged in rows and columns respectively, said first set being supported on said substrate; an electroluminescent layer interposed between said sets; a photoconductive layer interposed between said electroluminescent layer and said second set; a network of separated transparent conductors interposed between said photoconductive and electroluminescent layers, said transparent conductors being arranged in rows and columns which are in respective registration with said first and second sets; an opaque film having separated transparent areas defining rows and columns in registration with said first and second sets, said opaque film being interposed between said electroluminescent layer and said first set, said layers, said network and said film being electrically connected in series between said sets.

9. An electroluminescent device comprising first and second spaced apart electrode sets; an electroluminescent ceramic layer interposed between said sets; a photoconductive ceramic layer interposed between said electroluminescent layer and said second set; a network of separated transparent conductors interposed between said photoconductive and electroluminescent layers; and an opaque lacquer film having a plurality of separated transparent areas, each area being in registration with a corresponding transparent conductor, said film being interposed between said network and said photoconductive layer, said layers, said network and said film being electrically connected in series between said sets.

10. An electroluminescent device comprising first and second spaced apart electrode sets; an electroluminescent ceramic layer interposed between said sets; a photoconductive ceramic layer interposed between said electroluminescent layer and said second set; a network of separated transparent conductors interposed between said photoconductive and electroluminescent layers; an opaque lacquer film having a plurality of separated transparent areas, each area being in registration with a corresponding transparent conductor, said film being interposed between said electroluminescent layer and said first set, said layers, said network and said film being electrically connected in series between said sets; and a transparent glass substrate supporting said layers, said network, said film and said sets, said first set being adjacent said substrate.

11. In combination, an electroluminescent layer; a photoconductive layer; a network of separated transparent conductors interposed between said layers; an opaque film in contact with the surface of said electroluminescent layer remote from said network, said film having separated transparent areas, each area being in registration with a corresponding transparent conductor, a first electrode set in contact with said film; and a second electrode set in contact with the surface of said photoconductive layer remote from said network.

References Cited in the file of this patent UNITED STATES PATENTS 2,874,308 Livingston Feb. 17, 1959 2,877,371 Orthuber et al Mar. 10, 1959 2,882,419 Diemer et al Apr. 14, 1959 2,897,399 Garwin et a1 July 28, 1959 2,948,816 Van Santen et al Aug. 9, 1960 OTHER REFERENCES Proceedings of the I.R.E., pp. 1888 to 1897, December 1955.

groceedings of the I.R.E., pp. 1358 to 1364, October 19 7. 

